Monitoring and alarm systems are required for a wide variety of applications ranging from simple mechanisms to rather complex processes. An example of a simple mechanism requiring a monitoring and alarm system would be a home heating system, and an example of a complex process also requiring a monitoring and alarm system would be a petroleum cracking plant. In the past, the monitoring and alarm systems that have been provided for such diverse applications have been quite different reflecting the differing complexity of the applications. For example, a heating system might be equipped with a temperature sensor to monitor the plenum temperature of the furnace and a simple audio or visual alarm to provide an indication when a safe temperature is exceeded. In contrast, the petroleum cracking plant incorporates many processes that are mutually interdependent. Not only are temperatures at various points in the plant monitored, but flow rates, chemical constituents and various other variables are monitored. Some of the monitored variables may have single value set points or limits which, if exceeded, would constitute an alarm condition. More often, however, the variables being monitored are interdependent meaning that an alarm condition is not indicated unless a certain combination of variable values is detected.
The monitoring and alarm systems which have been developed for very complex applications are characterized by central processing units (CPU) connected to receive inputs from a plurality of sensors and to generate the appropriate alarms or other indications that may be required for the particular application. The CPU is programmed and otherwise adapted fo use in the specific environment. Since each installation is, in effect, a special purpose design, the monitoring and alarm systems for such complex applications are very expensive; however, the expense is justified by the relatively great cost of the application itself. There are on the other hand many applications which would be greatly improved by more sophisticated monitoring and alarm systems but for which the expense of such systems as presently designed cannot be justified.